In the search for high performance materials, considerable interest has been focused on carbon fibers and fabrics. The term "carbon" is used herein in its generic sense and includes graphite as well as amorphous carbon. Graphite fibers are defined herein as fibers which consist essentially of carbon and have a permanent x-ray diffraction pattern characteristic of graphite. Amorphous carbon fibers, on the other hand, are defined as fibers in which the bulk of the fiber weight can be attributed to carbon and which exhibit an essentially amorphous x-ray diffraction pattern. Graphite fibers generally have a higher Young's modulus than do amorphous carbon fibers and in addition are more electrically and thermally conductive. Carbon fabrics are sheet-like products formed from interwoven carbon fibers.
Industrial high performance materials of the future are projected to make substantial utilization of carbon-reinforced composites, and carbon fibers and fabrics theoretically have been described as providing desirable properties for high strength reinforcement. Among the desired properties are corrosion and high temperature resistance, low density, high tensile strength, and high modulus.
Reinforced components made from carbon graphite fibers have been observed to have limitations of undesirably low service ceiling temperatures, especially for applications using carbon graphite fibers exposed to the air or any oxidative environment. At temperatures as low as about 250.degree. C. carbon fibers are known to undergo thermally induced transformations which lead to degradation in strength. Reinforced components made from carbon graphite fibers, including carbon fiber/carbon fabric reinforced composites, can only be used up to a service ceiling temperature of about 400.degree. C. At temperatures approaching 400.degree. C. and above, the carbon fiber or fabric oxidizes, and the mechanical properties in the carbon composite degrade.
Technical efforts have been made to develop a thermal protection for the carbon fibers to increase this limited service ceiling temperature and to eliminate the erosion in properties which begins at about 250.degree. C. Currently, it is known to coat carbon graphite fibers with inorganic coatings using chemical vapor deposition such as by SiO.sub.2 vapor deposition, or by physical vapor deposition, as by nickel vapor deposition. However, the vapor deposition processes are found to be expensive, and further are found to be capable of application only to limited shapes and sizes of the fibers or substrates to be coated. Moreover, these vapor deposition methods would be incapable of uniformly coating fabric material, particularly in areas of interwoven fiber cross-over. The vapor deposition techniques also are known to have residual porosity, which porosity would provide centers of attack by the oxidative agents.
Accordingly, a process is needed for providing thermal and oxidative protection to carbon fibers or fabrics, or the resultant carbon-reinforced composites.
It is an object of the present invention to produce a polymer-coated carbon fiber or fabric having enhanced thermal protection and oxidative stability.
It is a further object of the present invention to produce a polymer-coated carbon fiber or fabric, or a resultant carbon-reinforced composite produced therefrom, in an economical and efficient manner and method, including for the application of the polymer coating.
It is another object of the present invention to provide a carbon fiber or fabric coated efficiently by a polymer coating process scaled up for any shape or size of carbon fiber or fabric to be coated.
It is an object of the present invention to provide a method for synthesizing coatings which confer enhanced thermal protection and oxidative stability to carbon fibers, fabrics, or substrates.
It is another object of the present invention to provide a product and method for providing thermal protection and oxidative stability to carbon fibers or fabrics without requiring high initial capital costs for producing the product or method.
It is a further object of the present invention to provide for coating carbon fibers or fabrics or to form a polymer-coated carbon or graphite fiber or fabric, or a resultant carbon-reinforced composite formed therefrom, having thermal protection and oxidative stability at temperatures higher than about 400.degree. C.
These and other objects of the present invention will become apparent from the detailed description which follows.